Why do you need to do this at all. I mean, usually there is a requisition created from the sales order and you just convert the PR into a PO. No need to enter the sales order number manually in the account assignment.
Maybe your sales order is on credit block and it is not allowed to procure anything.
That was as well part of the background why I asked if you are certain you have to use M instead of E, as these are 2 different processes which are referencing both a sales order.
What is your process?